The present invention relates to a method, and also to apparatus, for detecting the presence of predetermined hydrocarbon vapours in a monitored area. The invention is partcularly useful for detecting flammable hydrocarbon vapours when they are present at a concentration which might indicate a possible flammable or explosive atmosphere, and the invention is therefore described with respect to this application.
At the present time, analysis of an atmosphere to determine the concentration of a flammable-vapour therein is commonly carried out by combustion methods. The flammable vapour is fed (with air) into a combustion chamber where it is exposed to a controlled flame. When the vapour hits the flame, it burns and gives off heat which is sensed by a resistance temperature detector (RTD), or by a flame ionization detector (FID), which enables the concentration of flammable vapours to be determined.
Another technique, based on total organic-carbon (TOC) analysis, combines combustion with infrared absorption to determine the organic carbon content. The sample stream is oxidized to produce CO.sub.2, and the amount of CO.sub.2 is measured according to CO.sub.2 infrared analysis techniques.
However, the above known techniques of flammable-vapour analysis require exact calibration of the specific material to be detected, are very sensitive to other compounds with similar behaviour, require long time measurements (response time usually greater than one minute), are expensive and cumbersome, and/or lack versalitity in implementation.
It has also been proposed to detect a fire optically by the use of a visible wavelength band detector alone, or in combination with an infrared wavelength band detector (see for example U.S. Pat. No. 4,156,816 of May 29, 1979), and also to use a visible wavelength band detector for detecting various types of fluorocarbon gasses which may escape the environment (see for example U.S. Pat. No. 4,891,518 of Jan. 2, 1990). Other known types of vapour or gas analyzers based on the detection of radiation, or the attenuation of radiation by the gas or vapour being analyzed, are described in U.S. Pat. Nos. 4,385,516 of May 31, 1983, 4,517,161 of May 14, 1985 and 4,964,309 of Oct. 23, 1990. A further technique of analyzing a gas or vapour, based on measuring the strenght of acoustic resonance in a chamber containing a sample of the gas, is described in U.S. Pat. No. 4,055,764 of Oct. 25, 1977.